For example, in the process of manufacturing a 3D-NAND type nonvolatile semiconductor device, a process is performed that includes stacking a silicon oxide film (SiO2 film) and a silicon nitride film (SiN film) in multiple layers, forming a slit vertically in a stacking direction, wet-etching the SiN film via the slit to remove the SiN film, burying a tungsten film to be a gate electrode in a space formed after removing the SiN film, and then forming an SiO2 film (spacer SiO2 film) as a spacer insulating film of the tungsten film inside the vertical slit.
The process of forming the spacer SiO2 film is performed by ALD using oxygen active species, for example, plasma ALD. In order to prevent deterioration of electric characteristics or reduction of productivity due to the oxidation of tungsten and the volatilization of tungsten, a method has been used in which an ALD process at a low temperature is executed as a first process and then a ALD process at a high temperature is executed as a second process to improve a wet etching resistance.
The above first process is executed at a low temperature. Thus, it is unlikely that the tungsten film is directly oxidized or volatilized. However, oxygen active species such as oxygen radicals or the like infiltrate into the tungsten film. At that time, a natural oxide film (tungsten oxide) on the surface of the tungsten film also infiltrates into the tungsten film, thereby causing oxidation of the tungsten film.